1. Field of the Invention
The present invention relates to a pump that suctions and discharges fluid.
2. Description of Related Art
A fuel injection apparatus, which injects fuel to a compression ignition internal combustion engine, has a supply pump that compresses fuel and supplies the compressed fuel to a common rail. The supply pump has a hollow-cylindrical compression space (hereinafter, referred as a pump chamber) formed by an inner peripheral surface of a cylinder and an end surface (top portion) of a plunger. When the plunger reciprocates within the cylinder to pressurize fuel in the pump chamber, high pressure fuel is discharged toward the common rail through a discharge passage (for example, JP-A-S64-73166). For example, the discharge passage has an opening that is formed at an inner peripheral surface of the cylinder, which surface surrounds the pump chamber.
In the conventional supply pump, when fuel within the pump chamber is compressed, fuel pressure disadvantageously causes localized stress concentration generated around the opening formed at the inner peripheral surface of the cylinder.
Generation of the stress concentration at the opening formed at the inner peripheral surface of the cylinder will be described with reference to FIGS. 7A and 7B. FIG. 7A is a cross-sectional view of a part of a cylinder of the conventional supply pump, and FIG. 7B is a partial development for developing the vicinity of the opening of the cylinder inner peripheral surface in a circumferential direction along the inner peripheral surface of the cylinder of the conventional supply pump. It should be noted that multiple arrows in FIG. 7B indicate directions of tensile stress generated when fuel within the pump chamber is compressed.
The conventional supply pump, as shown in FIG. 7B, has an opening 130b. For example, the opening 130b has an oval shape and is formed at a cylinder inner peripheral surface 130a of a cylinder 130, which surface surrounds a pump chamber 150. The cylinder inner peripheral surface 130a intersects or is connected with an inner peripheral surface of a discharge passage 130c at the opening 130b as shown in FIG. 7A. When fuel in a pump chamber 150 is pressurized, fuel pressure expands the cylinder inner peripheral surface 130a, which surrounds the pump chamber 150, in a radially outward direction of the cylinder 130. Further, the discharge passage 130c is also expanded in a radially outward direction of the discharge passage 130c. As a result, an outline of the opening 130b formed at the cylinder inner peripheral surface 130a deforms from an oval shape (solid line in FIG. 7B) into a more circular shape (alternate long and short dash line in FIG. 7B).
In the above, tensile stress is applied to the cylinder inner peripheral surface 130a in a circumferential direction of the cylinder 130 along the cylinder inner peripheral surface 130a. Also, tensile stress is applied to the vicinity of the opening 130b, which has the oval shape, and which is formed at the cylinder inner peripheral surface 130a, in the circumferential direction of the discharge passage 130c along the opening 130b. 
In the above, tensile stress applied to the vicinity of the opening 130b is large at positions X (indicated by dashed line) and is small at positions Y (indicated by dashed line), and thereby distribution of tensile stress applied to the vicinity of the opening 130b is ununiform. As a result, localized stress concentration is more likely to be generated at the opening 130b of the cylinder inner peripheral surface 130a. Thus, repetition of suctioning and discharging fuel during the operation of the pump may cause fluctuation of stress at the vicinity of the opening 130b, and thereby fatigue failure may be caused disadvantageously. Subsequently, the cylinder may be broken.